Apparatus for drying coal in bunkers



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APPARATUS FOR DRYING COAL IN BUNKERS I Filed Feb. 25. 1961 5Sheets-Sheet 5 IN V EN TORS. .Pfl LPH CIPOE.

BY JQSEPHL zcuravsrs/m Arm/Mam United States Patent 3,181,488 APPARATUSFOR DRYING COAL 1N BEJ'NKERS Ralph C. Roe, Tenafly, NJ., and .iosephLichtenstein, Bayside, N.Y., assignors to Burns and Roe, Inc., New York,N.Y., a corporation of New Jersey Filed Feb, 23, 1961, Ser. No. 91,171 6Claims. (6!. 110-491) This invention relates to power plants in general,and more particularly to a method and apparatus for removing the surfacemoisture from coal in bunkers just prior to its being delivered to thefurnace for combustion therein.

Those persons skilled in the art will readily appreciate that thedifliculty experienced in the flow of coal through bunkers presents avery considerable problem. Thus, so called ratholing" or channeling andbridging frequently occur in the coal to interrupt its continuous flowto the furnace. It is well known in the art that this difficulty iscaused by surface moisture on the coal, geometry of bunker design and,to a relatively minor extent, surface roughness on bunker walls, withmoisture as the primary cause.

It has been found that even if the best bunker geometry is employed, itis most important to the continuous flow of the coal that surfacemoisture be eliminated; and in fact, the full extent of the problempresented by this moisture will only be appreciated when it is realizedthat if such moisture reaches the furnace, it has to be evaporated thereand the heat required to accomplish this is wasted, thus reducing theefficiency of the plant.

Heretofore, in attempting to dry coal prior to firing it has beenproposed to pass the hot gases of combustion through a jacketsurrounding a coal hopper or bunker, or such gases have been passedthrough the coal itself in direct contact with the coal particles. Eachof these methods is objectionable since, in the first case heating, andtherefore drying, is not uniform, the coal nearest the bunker wall beingheated while that in the center of the bunker receives little heat, ifany. In the second case, passing the hot combustion gases through thecoil has proven ineffective by virtue of the high moisture content ofthe gases themselves due to the release of water vapor upon combustionof the fuel. Also, some of the combustion gases such as sulphur dioxidein the presence of moisture are very corrosive at certain temperatures.

In essence, our invention resides in the recapture of otherwise wastedheat in the system, and the application of such heat to dry the coal asit moves through the bunkers, without any of the aforementioneddifficulties or disadvantages. Thus, we utilize waste heat from theexhaust furnace gases, usually after they have passed the normal airheater, and transfer the heat thus recaptured, to the coal in thebunkers by means of heat transfer fluid.

According to one aspect of the invention, a gas-to-liquid type heatexchanger is placed in the gas exhaust of a furnace and is connected bypiping and a pump to a series of spaced, vertically disposed, thin,hollow plates extending nearly the full height and length of eachbunker, and the entire system is filled with a heat transfer liquid,such as water under low pressure, for example. The fluid is heated bythe exhaust gases and pumped through the hollow plates contiguous withthe coal in the bunkers, whereupon the heat iiows by convection andradiation to the adjacent coal. Since the coal flows slowly from the topto the bottom of the bunkers, there is a sufficient period of time inwhich to heat the surface moisture on the coal to the temperaturenecessary to evaporate it. Of course, a certain amount of air isrequired to be supplied, either by natural convection, or by beingforced into the coal, to

carry the moisture from the surface of the coal to the atmosphere.

A further aspect of the invention involves a heat exchanger of theliquid-to-air type, positioned exterior of the bunkers whereby heat istransferred from the heat transfer liquid to air which is then forced,by means of a fan or blower, through a series of hollow air chambersspaced apart in the bunkers. Each chamber has suitable perforations sothat the heated air passes therefrom up through the coal, by reason ofthe void spaces between the coal particles, and evaporates the surfacemoisture thereon, carrying it out of the bunkers in the form ofincreased humidity of the air.

Still a further aspect of the invention contemplates the direct heatingof air by a gas-to-air heat exchanger in the path of flow of the furnaceexhaust gases. The air, thus heated by the exhaust gases, is deliveredto perforated air chambers in the bunkers from which it passes upthrough the coil, again to dry the same.

It is important in each of the several aspects of the invention alludedto that the air temperature within the bunkers be low enough to obviatethe danger of spontaneous combustion; and in each case, but particularlywhere air is discharged in the bunkers, the velocity of the air throughthe bunkers must be small enough to insure that even the finest coalparticles are not carried out into the atmosphere.

There has thus been outlined rather broadly the more important featuresof the invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described hereinafterand which will form the subject of the claims appended hereto. Thoseskilled in the art will appreciate that the conception upon which thisdisclosure is based may readily be utilized as a basis for the designingof other structures for carrying out the several purposes of theinvention. It is important, therefore, that the claims be regarded asincluding such equivalent constructions as do not depart from the spiritand scope of the invention.

A specific embodiment of the invention has been chosen for purposes ofillustration and description, and is shown in the accompanying drawings,forming a part of the specification, wherein:

FIG. 1 is a diagrammatic illustration of a system in accordance with thepresent invention;

FIG. 2 is a perspective view, partly broken away, of a bunker equippedwith a hot water coal drying assembly;

FIG. 3 is a perspective detail view of a hot water heater unit broken toindicate greater length;

FIG. 4 is a diagrammatic illustration similar to FIG. 1, but showing afurther embodiment of the present invention;

FIG. 5 is a perspective view, partly broken away, similar to FIG. 2, andshowing a bunker equipped with a heated air coal drying assembly;

FIG. 6 is a perspective view of a heated air unit; and

FIG. 7 is a diagrammatic illustration of a modification of theembodiment illustrated in FIG. 4.

Referring now to the drawings, and particularly to FIG. 1 thereof, thereis shown a system embodying the present concept and including a heattransfer unit 159 which preferably consists of a gas-to-liquid heatexchanger of the cast iron surface type positioned in the path ofcombustion gases from the furnace (not shown) to the stack 11. Asuitable heat exchange liquid which may be water under a pressure of theorder of about 250 lbs. per sq. in., to prevent its vaporization, ispumped through the heat transfer unit 10 by a pump 12 which ispositioned in a line 14 equipped with valving where necessary to conductthe heated fluid to a series of take-01f pipes as at 15 for deliverythrough manifolds 16 to a group of thin, hollow chamber-s or plates 17vertically positioned in spaced relation in coal bunkers 19. The heatedfluid passes through the plates 17 and is returned through valved returnmanifolds 20 and return line 21 to the heat transfer unit 1 0 forrecirculation.

The liquid system may be equipped with an expansion tank 22 to permitvariations in fluid volume in the system, and with a make-up line .24for maintaining the system suitably supplied with liquid.

As shown in FIGS. 2 and 3, the heated liquid flows from inlet manifolds16 into the upper portions of the plates 17 and leaves theplates at thebottom thereof via manifold 21). The plates are provided with a seriesof horizontal, spaced baffles 25 staggered longitudinally relatively toone another. As shown in FIG. 3, these baffies are alternately connectedto opposite end walls 26 of the plates 17 and each bafile extends thefull width of its respective plate. There is thus provided a tortuouspath for the heated liquid from end to end of each plate, each excursionbringing the liquid to a lower level, as indicated by the arrows in FIG.3 for example, until it reaches the outlet manifold 24) which isconnected to each plate beneath its lowermost baffle. Experienceindicates that generally, the plates 17 should be maintained a minimumof 24 inches apart. It is also preferred that the top surfaces of theplates 17 be rounded as shown to present a streamlined surface to thedownwardly flowing coal, the upper sloped surface of which is indicatedby the reference numeral 27 (FIG. 2).

It will be appreciated that the heat'carried by the liquid to the plates17 flows by both convection and radiation to the coal between theplates, and inasmuch as the coal flows slowly downwardly through thebunker, it is exposed to heat for a period of time quite sufficient forevaporation of its surface moisture. A certain amount of air is requiredas a vehicle to carry the moisture from the coal to the atmosphere.While natural convection may well suffice for this purpose, it is a partof the present concept to force air into the coal if desired in orderbetter to remove the water vapor. For this reason, a series of airchambers 2 are positioned substantially coextensive with and beneath theplates 17, and while such chambers may be independent of the plates,they are illustrated as being constructed integral therewith.Atmospheric air is supplied to the chambers 29 through air line 30, andthis air is discharged from the chambers in a manner and by means laterto be described in detail. For present purposes, it is sufficient tounderstand that the air thus discharged in the bunkers passes upwardlythrough the coal particles to carry the water vapor off to atmospherethrough vents 32 in the upper regions of the bunkers. While nine plates17 have been shown in FIG. 2, it will be understood that any number maybe employed as required in each specific instance, and it is of coursepreferred that the same number of chambers 29 be utilized.

Turning now to FIG. 4, there is illustrated a system similar to thatalready described, but with certain modifications. Thus, a heat transferunit it? is' positioned ahead of the stack '11 and the pump 12 pumpsa'heat'transfer liquid which may be water under pressure from the unit10 through pipe 14 and returns same to the unit 10 throughthe pipe 21,as before. The expansion tank 22 and make-up line 24 are also part ofthe present system.

However, according to the embodiment of the invention illustrated inFIG. 4, the liquid heated by the exhaust combustion gases is circulatedthrough a liquid-to-gas heat transfer unit 35 positioned between thepipes 14 and 21. Air under pressure supplied by a motor driven blower(not shown) is admitted to the unit 35 through air inlet 35 and leavesat hot air outlet 37 after having been heated by the liquid in the unit.The heated air passes through air lines 39, 40 and 41 to inlet manifolds42 from which it is delivered to ,a series of elongate, hollow airchambers 44 disposed in spaced horizontal relation within the bunkers19.

As shown in FIG. 5, the heated air is discharged from the chambers 44and comes into direct contact with the coal, the .air moving upwardlythrough the bunkers, as indicated by the arrows; The heated airevaporates the surface moisture from the coal particles, carrying suchmoisture off .as increased humidity. Vents 45 are provided in the top ofthe bunkers to allow for the escape of the air to atmosphere.

It is of course, important that the coal be prevented from clogging theoutlets of the chambers 44. Accordingly, as shown in FIG. 6, the bottomsurface 43 of each of these chambers is provided with a plurality ofperforations as at 46, and lower portions of the side and end walls 4-7and 49, respectively, depend downwardly to provide a peripheral flangeor skirt extending beyond the bottom surface 43. Thus, the coal passingdownwardly around the chambers will not come into contact with thebottom surfaces thereof, sothat the heated air is free to escape intothe voids in the coal and to move upwardly towards the vents 45. Again,the top surface of the chambers 44 are rounded or otherwise streamlined.The chambers 29 illustrated in FIG. 2 and referred to in accordance withan embodiment of the inventionaalready described may be constructedsimilarly to the chambers 44.

Referring finally to FIG. 7, there is shown a modification of theembodiment of the invention illustrated in FIG. 4. Thus, in this case,instead of the heat transfer units 10 and 35, a gas-to-air unit Si) ispositioned in the stream of combustion gases just ahead of the stack 11.An air compressor 51 supplies atmospheric air under pressure to the unit5%, and the compressed air, after being heated by the stack gases isconveyed by an air duct-52 to one or a number of bunkers via take-offducts 54. The bunker-s 11 'are'identica'l with those already describedand the heated air is discharged into the coal through a plurality ofhollow air chambers '44, as shown and described in connection with FIGS.4 to 6.

In a typical application of the present invention, 'consider a powerplant generating a million pounds of steam per hour or approximately 10Btu. per hour, and wherein 200,000 pounds per hour of coal enter thebunkers with average surface moisture equal to 8.72 percent of theweight of the coal. In this case, 17,450 pounds per hour of moistureenters the bunkers and must be removed' In accordance with the presentcontribution, air externally heated by means of warm water obtained in aheat transfer unit or economizer from the exhaust furnace gases inaccordance with the modification illustrated in FIG. 4 for example,could effect a fuel saving of approximately 2 percent, and this withoututilizing the warm air for furnace combustion. This saving alone issuflicient to pay for a substantial portion, if not all, of the requiredequipment, i.e., the heat transfer unit, air heater, motor, fan,circulating pump, piping and duct work. A preferred air velocity throughthe coal of about 19.6 feet per minute is effective to remove themoisture, and is so low that the air will notpick up even fine coalparticles and, therefore, would not create a dust nuisance or loss ofcoal.

'From the foregoing description, it will be seen that we havecontributed by our invention a novel system of drying coal in bunkers byutilizing the waste furnace gases and without any of the difliculties ordisadvantages attendant upon prior systems known to us.

'We believe that the construction and operation of our novel system willnow be understood and that the advantages of our invention will be fullyappreciated by those persons skilled in theart.

We now claim? 1. In a system of the class described for removing surfacemoisture from coal in bunkers prior to its introduction into a furnace,a heat exchanger positioned in heat exchange relation to the furnaceexhaust gases, means defining chambers respectively positioned withinthe bunkers immediately adjacent coal moving therethrough, and meansdirecting air under pressure through said heat exchanger to absorb heatfrom said exhaust gases and thence into said chambers, said chambershaving a plurality of apertures in the undersurfaces thereof fordischarging heated air into the natural void spaces between coalparticles passing through the bunkers to evaporate and carry olf asincreased humidity surface moisture from the coal, and means venting themoisture laden air from the bunkers,

2. In a system of the class described for removing surface moisture fromcoal in bunkers prior to its introduction into a furnace, a heatexchanger positioned in heat exchange relation to the furnace exhaustgases, means defining chambers respectively positioned within the lowerregion of the bunkers immediately adjacent coal moving downwardlytherethrough, means directing air under pressure through said heatexchanger to absorb heat from said exhaust gases and thence into saidchambers, said chambers having a bottom surface formed with a pluralityof downwardly facing apertures therein for dis charging heated air intothe natural void spaces between coal particles passing through thebunkers to evaporate and carry off as increased humidity surfacemoisture from the coal, said chambers also having a peripheral ependingskirt extending to a level below that of said bottom surface preventingthe downwardly moving coal particles from interfering with the flow ofair through said apertures, and means venting the moisture laden airfrom the bunkers.

3. In a system of the class described for removing surface moisture fromcoal in bunkers prior to its introduction into a furnace, means definingchambers positioned respectively in the bunkers for contact with coalparticles moving downwardly therethrough, a heat exchanger positioned inheat exchange relation to the furnace exhaust gases, means directing aheat transfer liquid through said heat exchanger to heat the liquid, atsecond heat exchanger, means directing a flow of air in heat exchangerelation to said heated liquid in said second heat exchanger and thenceto said chambers, said chambers having a bottom surface formed with aplurality of downwardly facing apertures therein for discharging theheated air into the natural void spaces between coal particles passingthrough the bunkers to evaporate and carry off as increased humiditysurface moisture from the coal, said chambers also having a peripheralskirt depending downwardly below the level of said apertures to preventthe downwardly moving coal particles from interfering with the flow ofair through said apertures, and means venting the moisture laden airfrom the bunkers.

4. In a system of the class described including a furnace, coal bunkersfor delivering coal to said furnace, and means for conducting exhaustgases from said furnace, the improvement that comprises a heat exchangerpositioned in said exhaust gas conducting means and in heat exchangerelation to the furnace exhaust gases to heat fluid therein, meansdefining a series of spaced thin fiat chambers positioned side-by-sidewithin the bunkers and extending from adjacent the top to adjacent thebottom and crosswise thereof immediately adjacent coal movingtherethrough, and means directing a heat transfer fluid through saidheat exchanger and thence through said chambers to transfer heat fromsaid chambers to the coal to evaporate surface moisture therefrom, aseries of vertically spaced horizontal bafile plates Within each of saidchambers providing a tortuous path for the heat exchange fluid as itmoves through said chambers thus to permit same to transfer heat byradiation and con vection to the bunker contents to effect evaporationof surface moisture therefrom, perforated air chambers extendingsubstantially coextensive with and: directly be low each of saidchambers to provide a flow of air upwardly through the coal to carry oifthe evaporated moisture and means at the top of the bunkers venting themoisture laden air.

5. A system according to claim 4, wherein the chambers are spaced atleast twenty-four inches apart.

6. A system according to claim 4, wherein the air is admitted to thebunkers at a rate to establish an air velocity through the coal of about19.6 feet per minute.

References Cited by the Examiner UNITED STATES PATENTS 1,608,699 11/26Kreisinge-r -106 2,033,685 3/36 Coutant 110-15 X 2,200,379 5 40 Williams3486 2,333,089 11/43 Burkhardt 34l77 2,458,412 1/49 Payne 34-168 X2,552,254 5/51 Brunschwyler 34177 X 2,642,314 6/53 Dupasquier 239-566 X2,706,343 4/55 Oholm 34-177X FOREIGN PATENTS 156,746 5/54 Australia.15,790 11/01 Sweden.

NORMAN YUDKOFF, Primary Examiner. BENJAMIN BENDETT, Examiner,

1. IN A SYSTEM OF THE CLASS DESCRIBED FOR REMOVING SURFACE MOISTURE FROMCOAL IN BUNKERS PRIOR TO ITS INTRODUCTION INTO A FURNACE, A HEATEXCHANGER POSITIONED IN HEAT EXCHANGE RELATION TO THE FURNACE EXHAUSTGASES, MEANS DEFINING CHAMBERS RESPECTIVELY POSITIONED WITHIN THEBUNKERS IMMEDIATELY ADJACENT COAL MOVING THERETHROUGH, AND MEANSDIRECTING AIR UNDER PRESSURE THROUGH SAID HEAT EXCHANGER TO ABSORB HEATFROM SAID EXHAUST GASES AND THENCE INTO SAID CHAMBERS, SAID CHAMBERSHAVING A PLURALITY OF APERTURES IN THE UNDERSURFACES THEREOF FORDISCHARGING HEATED AIR INTO THE NATURAL VOID SPACES BETWEEN COALPARTICLES PASSING THROUGH THE BUNKERS TO EVAPORATE COAL PARTICLESPASSING THROUGH THE BUNKERS TO MOISTURE FROM THE COAL, AND MEANS VENTINGTHE MOISTURE LADEN AIR FROM THE BUNKERS.